This application discloses an invention which is related, generally and in various embodiments, to a harmonic neutralized frequency changer (HFNC) control system and a HFNC including the same.
A Harmonic Neutralized Frequency Changer (HNFC) is a power converter system that produces variable frequency multiphase AC power from fixed or variable frequency AC power. Exemplary embodiments of an HNFC are described in U.S. Pat. No. 6,882,550 (the '550 patent), the contents of which are herein incorporated by reference in their entirety. Exemplary embodiments of a control system for an HNFC are described in U.S. Pat. No. 7,388,766 (the '766 patent), the contents of which are herein incorporated by reference in their entirety.
FIG. 1 illustrates a prior art HNFC, which is similar to the HNFC shown in FIG. 1 of the '766 patent. The prior art HNFC of FIG. 1 achieves a direct ac-to-ac power conversion using a cycloconverter (frequency change power converter). The unwanted harmonics from the cycloconverter are actively neutralized using high bandwidth PWM inverters (neutralization inverters). The prior art HNFC of FIG. 1 shows harmonic neutralization at both the input and output of the cycloconverter using series voltage injection with harmonic injection transformers. It will be appreciated that harmonic neutralization is also possible using shunt current injection as described, for example, at column 7, lines 14-45 of the '766 patent.
FIG. 2 illustrates a prior art HNFC control system for narrow band harmonic neutralization using series voltage injection at the output of the cycloconverter. The prior art HNFC control system of FIG. 2 is similar to the HFNC control system shown in FIG. 4 of the '766 patent. In the prior art HNFC control system of FIG. 2, the output voltage (Vo) is measured and fed back to multiple narrow band harmonic controllers. The narrow band harmonic controllers use harmonic numbers to select specific harmonic frequencies for neutralization. Each narrow band harmonic controller produces a set of inverter reference voltages (one for each electrical phase). The reference voltages from each narrow band harmonic controller are summed by phase to form a single set of three phase neutralization inverter reference voltages. The PWM inverter and its controller take the reference voltage signals and produce power voltages (neutralization waveforms) that actively cancel the harmonics in the output voltage waveform of the cycloconverter. The neutralization waveforms are added in series to the output voltage of the cycloconverter using a harmonic injection transformer. FIG. 2 shows a three-phase system for simplicity. Systems with multiple three-phase sets of output voltages are also covered in the '766 patent. In addition, the '766 patent also describes wide-band harmonic neutralization controllers using series voltage injection.
FIG. 3 illustrates a more detailed representation of the narrow band harmonic controllers of the prior art HNFC control system of FIG. 2. The detailed representation of the narrow band harmonic controllers shown in FIG. 3 is similar to the HFNC control system shown in FIG. 4 of the '766 patent. The measured output voltages (VAO, VBO, VCO) are converted from time domain to the rotating direct and quadrature axis reference frame using the equations in the bottom left corner of FIG. 3. The harmonic number h is multiplied by the reference angle α and used in the reference frame transformation to select the specific harmonic frequency targeted for neutralization. The harmonic reference voltages in the d-q reference frame are set to zero to eliminate the harmonic. Although equation 2, at column 4, lines 55-56 of the '766 patent may be read as implying that both positive and negative sequence harmonic components are possible (e.g., the positive sequence harmonic components “rotate” with the same sequence as the fundamental and the negative sequence harmonic components “rotate” in the opposite sequence as the fundamental), the '766 does not specifically describe the need to neutralize harmonic components of a given sequence (positive or negative). The voltage error signals in the d and q axes are operated on by proportional plus integral (P+I) controllers as shown to generate inverter reference voltages (Vqi*, Vdi*) in the rotating reference frame. The inverter reference voltages are transformed back to the time domain using the equations in the bottom right corner of FIG. 3. The 3-phase inverter reference voltages (VAh*, VBh*, VCh*) for each narrow band harmonic controller are sent to the summer shown in FIG. 2 and then on to the PWM inverter and its controller.
FIG. 4 illustrates a prior art HNFC control system for narrow band harmonic neutralization via shunt current injection at the output of the cycloconverter. The prior art HNFC control system of FIG. 4 is similar to the HFNC control system shown in FIG. 3 of the '766 patent. The output current (Io) is measured and fed back to multiple narrow band harmonic controllers. The narrow band harmonic controllers use harmonic numbers to select specific harmonic frequencies for neutralization. Each narrow band harmonic controller produces a set of inverter reference voltages (one for each electrical phase). The reference voltages from each narrow band harmonic controller are summed by phase to form a single set of three phase neutralization inverter reference voltages. The PWM inverter and its controller take the reference voltage signals and produce power voltages. The power voltages interact with a tie impedance to generate currents that actively cancel the harmonics in the output current waveform of the cycloconverter. The neutralization current waveforms are added directly in parallel to the output currents. FIG. 4 shows a three-phase system for simplicity. Systems with multiple three-phase sets of output voltages are also covered in the '766 patent. In addition, the '766 patent also describes wide-band harmonic neutralization controllers using shunt current injection.
FIG. 5 illustrates a more detailed representation of the narrow band harmonic controllers of the prior art HNFC control system of FIG. 4. The more detailed representation of the narrow band harmonic controllers shown in FIG. 5 is similar to the HFNC control system shown in FIG. 3 of the '766 patent. The operation of the current injection narrow band harmonic controller of FIG. 5 is similar to the voltage injection narrow band harmonic controller of FIG. 3. The measured output currents (IA, IB, IC) are converted from time domain to the rotating direct and quadrature axis reference frame using the equations in the bottom left corner of FIG. 5. The harmonic number h is multiplied by the reference angle α and used in the reference frame transformation to select the specific harmonic frequency targeted for neutralization. The harmonic reference currents in the d-q reference frame are set to zero to eliminate the harmonic. The voltage error signals in the d and q axes are operated on by proportional plus integral controllers as shown to generate inverter reference voltages (Vqh*, Vdh) in the rotating reference frame. The inverter reference voltages are transformed back to the time domain using the equations in the bottom right corner of FIG. 5. The 3-phase inverter reference voltages (VAh*, VBh*, VCh*) for each narrow band controller are sent to the summer shown in FIG. 4 and then on to the PWM inverter and its controller.